Add libvips support

[romanzks]

Feature: Add libvips support for image processing.
https://github.com/libvips/libvips
https://github.com/libvips/php-vips

It would be convenient, because now I have to use GD or Imagick to work with Color Thief PHP, while the rest of the processing is done by the libvips.

[ksubileau]

Thank you for your suggestion.

Libvips looks great, but I feel like it's not very commonly used, so I'm not sure it's worth adding official support for it right now. I'll probably wait to see if other people ask for this feature.

In the meantime, since the 2.0.0 release, it should still be possible to implement this support on your side, by providing an instance of a class implementing the ColorThief\Image\Adapter\AdapterInterface interface to the $adapter parameter of ColorThief::getColor and ColorThief::getPalette methods. Let me know if it works and can meet your need.

[romanzks]

Thanks for considering my suggestion and for the tip on the implementation of the functionality.